[74.02] The Infrared Emission Spectrum of Interstellar Dust

A. Li, B.T. Draine (Princeton University Observatory)

A substantial fraction -- ~1/3 -- of the observed
emission from interstellar dust in diffuse clouds is at
wavelengths \lambda\lesssim 50\mum. This short-wavelength
component of the emission is attributed to relatively small
grains which undergo ``temperature spikes" following
single-photon heating events. The observed emission contains
valuable information constraining both the composition and
the size distribution of interstellar dust.

We examine models consisting of two grain types -- amorphous
silicate grains and carbonaceous grains -- each with an
extended size distribution. The emission depends on the size
distributions and on the physical properties of the grains
-- cross sections for absorbing starlight photons, heat
capacities, and cross sections for thermal emission of
infrared photons. We assume that the carbonaceous grain
population extends from grains with graphitic properties at
radii a \gtrsim 0.01\mum, down to particles with PAH-like
properties at very small sizes. The optical properties of
the carbonaceous grains -- from the ultraviolet to the
infrared -- are based on laboratory studies. The optical
properties of the PAH-like particles include the prominent
IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.3\mum
with the effects of ionization taken into account. The size
distributions are constrained to be consistent with observed
interstellar extinction curves and with observed depletions
of C, Si, Fe, and other elements.

Since the temperature of a given grain is time-dependent,
the temperature distribution function for each grain type is
required. These temperature distribution functions have been
calculated, including both quantized heating and cooling.
The resulting emission spectra are compared to observations
from IRAS, COBE/FIRAS, COBE/DIRBE, IRTS, and ISO. Adjusting
the size distribution of the ultrasmall grain component, we
are able to reproduce the observed emission from the diffuse
interstellar medium with about 9% of the total C abundance
contained in the very small (a < 20Å) grains.
Furthermore, the maximum fraction of silicates which can be
present in very small particles is limited by the fact that
the 10\mum silicate emission feature is not detected in
emission. Our model spectra thus allow us to place an upper
limit on the abundance of a < 20Å\ silicate grains.

This research was supported in part by NASA grant NAG5-7030
and NSF grant AST-9619429.